Researchers looked at information provided by 54 male and 58 female middle and high school athletes in California who completed a questionnaire that asked about the number of sports they played, the amount of time they devoted to athletics, whether they used a private coach, whether they did strength training and how much sleep they averaged each night. The average age was 15.

The study authors then looked at school records about the students' sports injuries and found that athletes who slept eight or more hours each night were 68 percent less likely to be injured than those who regularly slept less.

The researchers also found that the higher the school grade level, the greater the risk of injury. It was 2.3 times greater for each additional grade. None of the other factors looked at in the study, including gender and number of hours played, was significantly associated with injury risk.

The findings were scheduled for presentation Sunday at the American Academy of Pediatrics' national conference in New Orleans.

"While other studies have shown that lack of sleep can affect cognitive skills and fine motor skills, nobody has really looked at this subject in terms of the adolescent athletic population," study author Dr. Matthew Milewski said in an AAP news release.

"When we started this study, we thought the amount of sports played, year-round play and increased specialization in sports would be much more important for injury risk," he said. Instead, "what we found is that the two most important facts were hours of sleep and grade in school."

The increased risk of injury among athletes in higher grades may be because they're bigger, faster and stronger, Milewski said.

Findings and conclusions presented at medical meetings should be considered preliminary until published in a peer-reviewed medical journal.